The present invention generally relates to an improved output circuit for use or with a differential line driver. The present invention more particularly relates to an improved output circuit which reduces kickback voltages produced by inductive loads coupled across the outputs of differential line drivers.
Differential line drivers are well known in the art and find many applications. One such application is in Ethernet Area Networks which include a main bus and which convey data packets in the form of Manchester encoded signals. These Manchester encoded signals are DC signals which have two levels, zero volt or minus two volts, for example. Differential line drivers serve the function in such networks to convert the DC Manchester encoded signals to AC signals.
The differential line drivers are used in tap transceivers which are stationed along the bus and are also used in serial interface adapters which, in one application, are coupled to the tap transceivers by a cable. Because the line drivers convert the DC signals to AC signals at their outputs, there is, except for one case, no net average DC level at their outputs. The only exception to this is in the case of an end of message (EOM) signal which is provided at the end of each data packet. The end of message signal is a positive voltage with a duration of two or three bit-times and is used to signal the end of a data packet.
The line driver of the tap transceiver is transformer coupled to the cable which, in turn, in accordance with the application referred to above, is transformer coupled to a serial interface adapter. The equivalent load to the line driver output includes a parallel coupled load inductance and a line termination load resistance across the line driver output terminals. During the normal transmission of a data packet, there is no average DC level at the line driver outputs. However, during the end of message signal, there is an average DC level which results in a residual magnetizing current in the load inductance. After the end of message signal is terminated, the residual magnetizing current flows through the load resistor causing an undershoot or kickback voltage of reverse polarity across the load resistance and, as a result, across the line driver output terminals. If this kickback voltage is too high, it could be falsely interpreted as the beginning of a new data packet. Obviously, such kickback voltages are undesirable.
Because such kickback voltages are undesirable, it is generally required to maintain the maximum magnitude of the kickback voltage to less than 100 millivolts. In order to meet this requirement, some line drivers have employed additional resistive loading across the line driver output terminals. Unfortunately, this limits the output voltage otherwise obtainable with these line drivers. Also, since the output inductance loading may vary depending upon the applications, some line drivers are not capable of meeting the kickback voltage requirement for all applications.
The present invention overcomes the aforementioned difficulties with respect to kickback voltage by providing an output circuit for a differential line driver which substantially reduces the kickback voltage. The output circuit of the present invention accomplishes this end by allowing only a portion of the residual magnetizing current to flow through the load resistance at the termination of an end of message signal. The output circuit of the present invention may be implemented externally to existing integrated circuit line drivers by employing discreet components, and, since the output circuit of the present invention incorporates integrated circuit processing compatible components, it may also be integrated into an integrated circuit along with the line driver with which it is associated.